In high intensity arc discharge lamps, the radiated light output is derived from a plasma arc discharge within an arc tube. One form of high intensity discharge lamp that is currently and conventionally employed is the metal halide lamp. In such lamps the arc discharge tube includes a metal halide, such as sodium iodide, which is vaporized and dissociated in the plasma arc during lamp operation. However, in the vicinity of the arc tube walls, where the temperature is cooler, sodium remains chemically bound to the iodide preventing the sodium from absorbing some of the light radiation. Without the added halide, the self-absorption characteristics of cooler sodium atoms distributed preferentially near the cooler arc tube walls would act to limit lamp efficacy. In particular, sodium D-line radiation produced within the hot central plasma region of the arc tube would be readily absorbed by the cooler sodium atoms which would be present near the arc tube walls.
While the addition of halides to the lamp reduces the presence of free sodium near the cooler arc tube walls, it also requires a buffer gas to limit the transport of energy from the hot core of the arc to the arc tube walls via chemical reaction. The conventional use of mercury to buffer the chemical transport of energy from the plasma arc to the arc tube walls requires very high mercury pressures. However, the use of high pressure mercury asymmetrically broadens the sodium D-line on the red side, enhancing non-efficacious radiation output. Further reduction of observed efficacy is presumed to be caused by the tying-up of iodine by the large excess of mercury buffer gas, especially in the cooler parts of the arc tube where mercury iodide is stable. It would be desirable to eliminate these drawbacks in high pressure sodium iodide arc discharge lamps.